1. Field of the Invention
The invention relates to a controller for controlling an air-fuel ratio based on the output of an exhaust gas sensor disposed in an exhaust system of an internal-combustion engine.
2. Description of the Related Art
A catalyst converter is provided in an exhaust system of an internal combustion engine of a vehicle. When the air-fuel ratio of air-fuel mixture introduced into the engine is lean, the catalyst converter oxidizes HC and CO with excessive oxygen included in the exhaust gas. When the air-fuel ratio is rich, the catalyst converter reduces NOx with HC and CO. When the air-fuel ratio is in the stoichiometric air-fuel ratio region, HC, CO and NOx are simultaneously and effectively purified.
An exhaust gas sensor is provided downstream of the catalyst converter. The exhaust gas sensor detects the concentration of oxygen included in the gas that is discharged into the exhaust system. Feedback control for the air-fuel ratio of the engine is performed based on the output of the exhaust gas sensor.
As an example of the feedback control for the air-fuel ratio, Japanese Patent Application Unexamined Publication No. 2000-234550 proposes a response assignment control in which a switching function is defined. This control converges the output of the exhaust gas sensor to a desired value by converging the value of the switching function to zero. A desired air-fuel ratio (or manipulated variable) for converging the output of the exhaust gas sensor to the desired value is calculated. A fuel amount to be supplied to the engine is controlled according to the desired air-fuel ratio.
A system identifier may be provided in a system that performs the response assignment control. The system identifier calculates model parameters associated with an object of the response assignment control. The model parameters calculated by the system identifier are used to determine the desired air-fuel ratio.
Recently, there is a trend to expand an operating range in which the engine is operated with a lean air-fuel ratio so as to improve fuel efficiency. When a desired engine operation cannot be achieved with a lean air-fuel ratio, the air-fuel ratio is changed to the stoichiometric air-fuel ratio or a rich air-fuel ratio. When the engine is operated with the stoichiometric air-fuel ratio, air-fuel ratio control according to the above response assignment control is performed so as to reduce the emission of undesired substances contained in exhaust gas.
Engine operation with a lean air-fuel ratio may be also activated immediately after the engine is started. Such lean engine operation is performed so as to reduce the emission of undesired substances contained in exhaust gas.
According to a conventional air-fuel ratio control, only in lean engine operation activated immediately after the engine is started, the calculation of the model parameters by the identifier is stopped. In lean engine operation activated so as to improve fuel efficiency, the identifier continues calculating the model parameters, and the calculation of the desired air-fuel ratio by using the calculated model parameters is stopped.
FIG. 14 shows behavior of parameters according to such a conventional air-fuel ratio control. An exhaust gas sensor output Vo2/OUT, model parameters a1 and a2, a desired air fuel ratio KCMD, an actual air-fuel ratio KACT, and the amount of undesired substances HC and NOx contained in exhaust gas are shown.
During engine operation with a lean air-fuel ratio (t1 to t2) and immediately after the lean engine operation (t2 to t4), the exhaust gas sensor output Vo2/OUT and the actual air-fuel ratio KACT exhibit a lean air-fuel ratio. During a period from t1 to t4, the identifier continues calculating the model parameters a1 and a2 based on the exhaust gas sensor output Vo2/OUT and the actual air fuel ratio KACT. Since the exhaust gas sensor output Vo2/OUT and the actual air fuel ratio KACT have a constant lean air-fuel ratio, the accuracy of identifying the model parameters a1 and a2 deteriorates. The model parameters drift as shown in the period from t2 to t4.
The desired air fuel ratio KCMD is held at a predetermined value (for example, 1) during the lean engine operation (t1 to t2). At time t2 at which the lean engine operation is terminated, an adaptive air-fuel ratio control is started and the calculation of the desired air fuel ratio KCMD is also started.
During a period from t2 to t3, the desired air-fuel ratio needs to be manipulated to become rich so as to promptly return the output of the exhaust gas sensor from the lean side to the desired value Vo2/TARGET. However, due to the drift of the model parameters, the desired air-fuel ratio KCMD is changed toward the lean side as shown by reference number 201. As a result, the air-fuel ratio is manipulated to converge to the lean desired air-fuel ratio KCMD, thereby increasing Nox emission.
During a period from t3 to t4, the desired air-fuel ratio needs to be manipulated to change toward the lean side so as to cause the output of the exhaust gas sensor to converge to the desired value Vo2/TARGET. However, due to the drift of the model parameters, the desired air-fuel ratio KCMD is changed toward the rich side as shown by reference number 202. As a result, the air-fuel ratio is manipulated to converge to the rich desired air-fuel ratio KCMD, thereby increasing HC emission.
Thus, as shown in the period from t2 to t4, drift of the model parameters may make the calculation of the desired air-fuel ratio KCMD inappropriate. An inappropriate desired air-fuel ratio increases NOx and HC. Such increase of NOx and HC may also occur when fuel-cut operation that stops fuel supply to the engine is performed.
Therefore, there is a need for an apparatus and a method capable of stopping the identifier from calculating the model parameters during and immediately after such lean engine operation and fuel-cut operation.